Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids

TL;DR

This paper proposes a theoretical framework supported by cosmological simulations explaining the formation of massive high-redshift galaxies through cold streams, disk instability, and bulge growth, predicting a bimodal galaxy evolution.

Contribution

It introduces a new simple theoretical model for high-redshift galaxy formation, validated by first cosmological simulations showing clumpy disks and their evolution.

Findings

Clumpy disks are sustained by smooth cold streams and gravitational instability.

Giant clumps migrate inward to form bulges within ~0.5 Gyr.

Galaxy bimodality emerges by redshift ~3, with star-forming disks and passive spheroids.

Abstract

We present a simple theoretical framework for massive galaxies at high redshift, where the main assembly and star formation occurred, and report on the first cosmological simulations that reveal clumpy disks consistent with our analysis. The evolution is governed by the interplay between smooth and clumpy cold streams, disk instability, and bulge formation. Intense, relatively smooth streams maintain an unstable dense gas-rich disk. Instability with high turbulence and giant clumps, each a few percent of the disk mass, is self-regulated by gravitational interactions within the disk. The clumps migrate into a bulge in ~10 dynamical times, or ~0.5Gyr. The cosmological streams replenish the draining disk and prolong the clumpy phase to several Gigayears in a steady state, with comparable masses in disk, bulge, and dark matter within the disk radius. The clumps form stars in dense subclumps following the overall accretion rate, ~100 Msun/yr, and each clump converts into stars in ~0.5 Gyr. While the clumps coalesce dissipatively to a compact bulge, the star-forming disk is extended because the incoming streams keep the outer disk dense and susceptible to instability and because of angular momentum transport. Passive spheroid-dominated galaxies form when the streams are more clumpy: the external clumps merge into a massive bulge and stir up disk turbulence that stabilize the disk and suppress in situ clump and star formation. We predict a bimodality in galaxy type by z~3, involving giant-clump star-forming disks and spheroid-dominated galaxies of suppressed star formation. After z~1, the disks tend to be stabilized by the dominant stellar disks and bulges. Most of the high-z massive disks are likely to end up as today's early-type galaxies.